Lithography, particularly photolithography, is used to fabricate semiconductor-integrated electrical circuits; integrated optical, magnetic, mechanical circuits; and microdevices. Lithographic pattern formation involves chemically treating specific regions of a thin film carried on a substrate then removing either the treated or untreated regions as appropriate, for example, by dissolving in a processing solvent. In subsequent steps, the pattern is replicated in the substrate or in another material. In combination with traditional resist imaging, lithography can be used to manufacture printing plates and resist images. The thin film, which accepts a pattern or image during the lithographic process, is often referred to as resist. The resist may be either a positive resist or a negative resist. A positive photoresist becomes more soluble in the processing solvent where irradiated, while a negative resist becomes insoluble where irradiated. A typical lithographic process for integrated circuit fabrication involves exposing or irradiating a photoresist composition or film with a radiation or particle beam, such as light, energetic particles (e.g., electrons), photons, or ions by either passing a flood beam through a mask or scanning a focused beam. The radiation or particle beam changes the chemical structure of the exposed area of the film, so that when washed or immersed in a processing solvent, either the exposed or the unexposed areas of the resist dissolve. Lithographic resolution is limited by the wavelength of the particles, the resolution of the beam, the particle scattering in the resist and the substrate, and the properties of the resist. There is an ongoing need in art of lithography to produce smaller pattern sizes while maintaining cost efficiency. Particularly, there is a great need to develop low-cost technologies for mass-producing sub-50 nm structures. As used herein, the term “sub-xx nm features”, wherein xx is a number, generally refers to a plurality of structures having at least one dimension less than xx nm. As used herein, the term “sub-xx nm features”, wherein xx is a number, refers generally to a plurality of structures having at least one dimension less than xx nm. Such developments will have an enormous impact in many areas of engineering and science.
Numerous technologies have been developed to service these needs, but they all suffer drawbacks and cannot be used to mass produce sub-50 nm lithography at a low cost. Electron beam lithography has demonstrated 10 nm lithography resolutions. A. N. Broers, J. M. Harper, and W. W. Molzen, APPL. PHYS. LETT. 33, 392 (1978) and P. B. Fischer and S. Y. Chou, APPL. PHYS. LETT. 62, 2989 (1993). But using this technology to mass produce sub-50 nm structures is economically impractical due to inherent low throughput. X-ray lithography, which can have a high throughput, has demonstrated 50 nm lithography resolution. K. Early, M. L. Schattenburg, and H. I. Smith, MICROELECTRONIC ENGINEERING 11, 317 (1990). But X-ray lithography devices are expensive. X-ray lithography has not been used to commercially mass produce sub-50 nm structures. Lithography based on scanning probes has produced sub-10 nm structures in a very thin layer of materials. But, the practicality of such lithography as a manufacturing tool is not apparent.
Another nanostructure manufacturing process is refereed to in the art as nanoimprinting or nanoimprint lithography, which involves compressive patterning of deformable films coated on a substrate by way of a mold having protrusions and recesses. See for example, U.S. Pat. Nos. 5,772,905 and 6,309,580. The thickness of the film under the protruding feature is thinner than the thickness of the film under the recess. Thus, a relief is formed in the thin film. The relief conforms the mold's features. The relief is processed such that the thinner portion of the film is removed thereby exposing the underlying substrate in a pattern complementary to the mold. The relief patterns so produced can be reproduced in the substrate or in another material.
The patterns formed in nanoimprint lithography are defined by the mold instead of any radiation exposure. Nanoimprint lithography can eliminate many resolution limitations imposed in conventional lithography, such as wavelength limitation, backscattering of particles in the resist and substrate, and optical interference.
This low-cost mass manufacturing technology and has been around for several decades. Using nanoimprint technology, features on the order of 1 micrometer have been routinely imprinted in plastics. Compact disks, which are based on imprinting of polycarbonate, are one example of the commercial use of this technology. Other examples are imprinted polymethylmethacrylate (PMMA) structures with a feature size on the order to 10 micrometers for making micromechanical parts. M. Harmening et al., PROCEEDINGS IEEE MICRO ELECTRO MECHANICAL SYSTEMS, 202 (1992). Molded polyester micromechanical parts with feature dimensions of several tens of microns have also been used. H. Li and S. D. Senturia, PROCEEDINGS OF 1992 13TH IEEE/CHMT INTERNATIONAL ELECTRONIC MANUFACTURING TECHNOLOGY SYMPOSIUM, 145 (1992). But imprint technology has not been able to provide 25 nm structures with high aspect ratios.
Because nanoimprint lithography is based on the deformation of the polymer resists by a mold instead of changing the solvent-dissolution properties of the resists in photolithography (E. Reichmanis and L. F. Thompson, CHEM. REV. 89, 1273-1289 (1989)), it is necessary to develop the specific polymer resist compositions that can be easily deformed with good viscose flow ability by mold on a substrate and can survive on the substrate after mold separation. Disadvantageously, the thin-film compositions used in standard nanoimprinting processes have physical properties that cause deformities that decrease resolution. Stress is caused when higher temperatures are used to increase the polymeric film's flowability so that it can flow into the nanomold. As used herein, the term “nanomold” generally refers to a mold having a plurality of structures having at least one dimension less than 200 nm. On the other hand, if the temperature used during heated-imprinting is not too high and the resist material is then cooled and solidified after conformal deformation against the mold, or if other physical or chemical conditions are applied after conformal imprinting of a liquid resist material at room temperature and the material is solidified, then the resist material does accurately conform to the small features of the mold because of the decreased or totally loss of the flowability.
The requirements for nanoimprint lithography materials (“nanoimprint resists”) are quite different than polymeric materials that are typically used in traditional plastics molding techniques, such as injection molding or liquid casting. For example, nanoimprint resists typically require the ability to be processed into uniform thin-films on substrates. In addition, the rheology (i.e., flow characteristics) of polymeric materials deposited as thin polymeric films or discrete liquid drops on surfaces is oftentimes quite different that the rheology of bulk polymeric materials or liquids.
U.S. Pat. No. 5,772,905 discloses the use of polymethylmethacrylate (“PMMA”) as a nanoimprint resist, which is advantageously spin castable on a silicon wafer, has good mold release properties and has low thermal shrinkage. The disclosed nanoimprint process requires heating of the spin coated PMMA nanoimprint resist to temperatures (ca. 200° C.) substantially higher than the glass transition temperature (“Tg”) of PMMA (ca. 105° C.) to soften the resist to enable nanoimprinting. The nanoimprint mold is removed after cooling the nanoimprint resist below Tg. This heating and cooling disadvantageously requires process time and can lead to alignment and registration problems of the process equipment arising from thermal expansion and contraction. The need therefore exists to develop nanoimprint resists that overcome these problems.
U.S. Pat. No. 6,309,580 the discloses nanoimprint lithography wherein the mold is pre-treated with a release material that facilitate mold removal and thereby enhance image resolution. Use of the release material also protects the mold so that it can be used repeatedly without showing wear of its fine features. After the relief is processed, the exposed portions of the substrate's surface have sub-200 nm features. Because mold pretreatment is an additional step that is preferably eliminated from the nanoimprint lithography process to increase manufacturing throughput, the need therefore exists to develop nanoimprint resists that provide enhanced image resolution without the need to pretreat the nanomolds.
Accordingly, there is a continuing need for additional improvements in processes, apparatus, materials, and protocols for use in nanoimprint lithography. For example, there is need for new thin-film compositions for use in nanoimprint technology that overcome the above-mentioned problems. Thus, there is a need to provide nanoimprint resists that do not require extensive heating and cooling and which release well from untreated molds.
Mostly, the ultimate goal of the lithography process is to make 3D shapes out of certain functional materials. In today's art, almost all the lithography methods (photolithography, electron-beam lithography, including nanoimprint lithography) are used to first define the micro- or nano-patterns on top of a functional material. To finally achieve 3D shapes of the functional material, subsequent steps (often being multiple steps) are needed to remove and shape the materials. This is obviously a costly process. Imprinting provides the advantage of directly shaping a material into 3D structures; and many functional materials used in micro- and nano-devices are inherently moldable or can be redesigned and formulated to be moldable. These type of materials include, but not limited to, dielectric materials, conductive polymers, organic LED materials, optical media, photoactive materials, and chemically active materials. Therefore, nanoimprint process can be used to make functional material structures in an essentially one-step process, greatly saving manufacturing cost of these types of devices. Accordingly, new imprintable functional material compositions are needed.